1. Field of the Invention
This invention pertains to imaging systems in general, and in particular, to a method and apparatus for mounting a solid-state optical sensor array to, and in optical alignment with, a lens or other image-forming element of an optical device.
2. Related Art
The advent of simple, relatively inexpensive semiconductor, or solid-state, optical sensors has wrought a change in the way light images are captured, manipulated, broadcast, reproduced, and displayed. The last decade or so has seen a proliferation in the variety of devices incorporating such sensors that are available, not only to high-end users, such as professional video studios and graphics art houses, but to ordinary consumers as well. Such devices includes video cameras, digital still cameras, desktop scanners, film scanners, bar-code readers, security scanners and the like, that are capable of capturing relatively high resolution monochrome or color images, and converting them into analog or digital signals for storage, manipulation and/or distribution.
Such devices ordinarily comprise a lens or other image-forming element capable of capturing the light from a scene or subject and focusing or projecting that light onto a surface that is capable of sensing the light. This surface typically comprises an array of tiny photosensor elements, such as charge-coupled-devices ("CCDs") or complementary metal oxide semiconductor ("CMOS") photoreceptors. Alternatively, the photosensitive surface might comprise a light-reflecting surface, such as the electrostatic "micro-mirror light valve" described in U.S. Pat. No. 5,768,009 to M. J. Little.
These sensors typically comprise planar, rectangular matrices, or arrays, of photoelectric transducer elements fabricated on the surface of semiconductor substrates, typically silicon, by known photolithographic techniques, that are capable of converting the light energy incident upon them into electrical signals on an element-by-element, or "pixel"-by-"pixel," basis. These signals, usually digital in nature, include information pertaining to, e.g., the intensity, color, hue, saturation, and other attributes of the incident light. Examples of such semiconductor photosensor arrays can be found in the "CMOS active pixel image sensor array" described by B. D. Ackland et al. in U.S. Pat. No. 5,835,141, the "staring array detectors (`SADs`)" described by R. S. Holcomb in U.S. Pat. No. 5,864,132, and the "single chip color MOS image sensor" described by D. Chen et al. in U.S. Pat. No. 5,901,257.
The sensor array substrates are typically individually packaged in a hermetically sealed package having signal input/output terminals and a clear glass or plastic lid, or window, that exposes the light-sensitive elements of the sensor below it to the incident light. One such sensor package, known commercially as the Visionpak,.TM. is described in detail in co-pending U.S. application Ser. No. 08/844,536, filed Apr. 18, 1997, and owned by the same proprietor as this invention.
A common requirement in the assembly of imaging devices such as those described above is that the sensor array be in accurate optical alignment with the lens or other image-forming element of the device in six degrees of measurement, so that the image signal produced by the sensor accurately represents the light information present in the scene imaged by the lens. In particular, the plane of the sensor array must be coplanar with the focal plane of the lens, or the image will be out of focus, the center of the sensor array must be centered on the optical axis of the lens, or the image will be off center or only partially sensed, and the horizontal and vertical axes of the rectangular array must be aligned with the horizontal and vertical axes of the imaged scene, or the image will be canted relative to the actual scene. While some misalignment between the lens and the sensor can be tolerated and/or compensated for electronically by signal processing techniques, it is usually both necessary and preferable to align the sensor optically with the lens during their assembly with as much precision as is cost-justified.
There are generally two methods of aligning an optical sensor to the lens, or image-forming element, of an optical device: A "custom" or "closed-loop" method, and a "blind," or "open-loop" method. In the former, the sensor is temporarily positioned loosely in about the desired position of alignment with the lens, and the sensor is temporarily connected to a display, e.g., a cathode ray tube ("CRT"). The lens then images a test scene or pattern, and the position of the sensor is manually adjusted relative to the lens by a human operator until the image of the test pattern produced on the display subjectively matches the test pattern, whereupon the position of the sensor relative to the lens is then fixed permanently in place, e.g., by screws or an adhesive. Because this method is relatively labor-intensive and requires a skilled operator, it is also relatively expensive, and hence, is typically reserved for relatively expensive devices, such as professional-grade video cameras. Examples of systems in which this "closed loop" method is employed may be found in the scanner mount of U.S. Pat. No. 4,457,017 to T. Onogi et al.; the adjustable mount for a video camera of U.S. Pat. No. 4,803,557 to M. E. Bridges; and in the television camera of U.S. Pat. No. 4,591,901 to Z. M. Andrevski.
The "blind," or "open-loop," method, on the other hand, involves forming or mounting a first fixture on the lens in precise alignment relative to certain optical features of the lens, e.g., its optical axis and its focal plane. The sensor is mounted on a second fixture such that the optical features of the sensor array, e.g., its optical center, are in precise alignment relative to the second fixture. The two respective fixtures have corresponding, complementary mounting features adapted to engage each other such that, when engaged, the optical features of the sensor are aligned with the optical features of the lens.
Typically, the first fixture comprises projections or orthogonal ledges arranged around the optical axis of the lens, whereas, the second fixture comprises corresponding apertures in the package of the sensor, or more typically, in a printed circuit board ("PCB") to which the sensor package is mounted. Applications involving the blind alignment method may be found in the solid state imaging device assembly of U.S. Pat. No. 4,594,613 to K. Shinbori, et al.; in the circuit board/sensor alignment apparatus described in U.S. Pat. No. 5,559,556 to T. Kagebeck; in the film gate apparatus for a color film scanner described in U.S. Pat. No. 5,267,043 to B. E. Rottner, et al.; and the film scanner solid state sensor mount described in U.S. Pat. No. 5,828,409 to S. P. North, et al.
Because the blind method avoids the labor-intensive, trial-and-error techniques of the "custom" method described above, it is typically less expensive to implement on a volume basis than the latter method, and thus makes low-to-medium-cost devices more practicable, e.g., low-end scanners, and still or video cameras. However, because of the potential for tolerance buildup in the fabrication and assembly of the various components of the devices prior to their complete integration, particularly in those present in the solid state sensor itself, it is also typically less accurate than the custom method, unless extremely tight dimensional tolerances are specified and maintained during the fabrication and assembly of all of the components in the "chain" between the sensor and the lens.
The sources of this tolerance build-up can be broken down into two groups: Those "direct" tolerances associated with the fabrication of the individual components, and those "relative" tolerances associated with the assembly of the components to each other. The first group includes the tolerances associated with the sensor array itself, the sensor chip, the chip package, the PCB to which the package is mounted, the sensor-engaging fixture on the lens, and the lens itself. The second group includes the relative tolerances between the sensor array and the chip, the chip and the chip package, the chip package and the PCB, the PCB and the sensor-engaging fixture, and sensor-engaging fixture and the lens.
Because maintaining extremely small dimensional tolerances throughout this entire chain of components is both difficult and expensive, such a requirement can more than offset the cost advantage of the blind alignment method.
What is needed, then, is a simple, low-cost method of blind mounting a solid state optical sensor to a lens or other imaging-forming element of an optical device in relatively precise optical alignment therewith that bypasses some of the above sources of tolerance buildup, so that the need for holding extremely tight mechanical tolerances throughout the assembly chain of the component parts of the device is eliminated.